1. Field of the Invention
The invention relates to a speed reduction mechanism that is suitable for use in, for example, an electric vehicle including an electric motor as a drive source, and also relates to motor torque transmission apparatus including the speed reduction mechanism.
2. Description of Related Art
A conventional motor torque transmission apparatus includes an electric motor and a reduction-transmission mechanism. The electric motor generates motor torque. The reduction-transmission mechanism transmits driving force based on the motor torque of the electric motor, to a differential mechanism. The motor torque transmission apparatus is installed in, for example, an automobile (refer to, for example, Japanese Patent Application Publication No. 2007-218407 (JP2007-218407 A)).
The electric motor has a motor shaft that is rotated by electric power from an onboard battery. The electric motor is arranged on the axis of the reduction-transmission mechanism.
The reduction-transmission mechanism has a shaft portion (rotary shaft with eccentric portions) and a pair of reduction-transmission portions. The shaft portion is spline-fitted to the motor shaft of the electric motor. The reduction-transmission portions are arranged around the rotary shaft with the eccentric portions. The reduction-transmission mechanism is interposed between the electric motor and the differential mechanism (differential case). Further, the reduction-transmission mechanism is coupled to the motor shaft and the differential case. Moreover, the reduction-transmission mechanism is accommodated in a housing together with the electric motor and the differential mechanism.
With the configuration as described above, the motor shaft of the electric motor is rotated by electric power from the onboard battery, and as a result, motor torque is transmitted from the electric motor to the differential mechanism via the reduction-transmission mechanism, and is then distributed to right and left wheels by the differential mechanism.
The reduction-transmission portions of the motor torque transmission apparatus of the above-mentioned type include a pair of disc-shaped revolving members, a plurality of outer pins and a plurality of inner pins. The revolving members make revolving motions in accordance with rotation (rotation of the rotary shaft with the eccentric portions) of the motor shaft of the electric motor. The outer pins apply rotation force to the revolving members. The inner pins arranged radially inward of the outer pins output the rotation force of the revolving members in the form of torque, to the differential mechanism.
Each of the revolving members has a center hole and a plurality of pin insertion holes. The center hole opens toward both sides in the direction of its central axis. The pin insertion holes are arranged around the central axis of the center hole at equal angular intervals. The revolving members are rotataby supported on the eccentric portions of the rotary shaft with the eccentric portions, via bearings (cam-side bearings).
The outer pins are arranged around the axis of the motor shaft at equal angular intervals. Further, the outer pins are fitted to a housing of the reduction-transmission mechanism.
The inner pins are extended through the pin insertion holes of the revolving members. The inner pins are arranged around the axis of the motor shaft at equal angular intervals. Further, the inner pins are fitted to the differential case. Bearings (pin-side bearings) are fitted to the inner pins. The bearings are used to reduce contact resistance between the inner pins and the inner peripheries of the revolving members, which define the pin insertion holes.
In the motor torque transmission apparatus described in JP 2007-218407 A, a plurality of the outer pins needs to be prepared, and further, the outer peripheral portions of the revolving members need to be formed into a complex shape, which is uneconomical.
To avoid such a problem, external gears may be employed as the revolving members, an internal gear may be employed as the rotation force applying member that applies rotation force to the revolving members, and the number of teeth of the internal gear may be set larger than that of each external gear.
However, when the reduction-transmission mechanism including the external gears and the internal gear is used in motor torque transmission apparatus for an automobile, if a load caused by a centrifugal force is exerted on a differential mechanism-side end portion of the motor shaft (rotary shaft) from the external gears during output of the power, the rotary shaft may be slanted or runout of the rotary shaft may occur. If the rotary shaft is slanted or runout of the rotary shaft occurs, the external gears are meshed with the internal gear while being slanted, and therefore, off-center between the internal gear (rotation force applying member) and the output target is caused, resulting in hindrance to smooth rotation of the differential case.